The use of power amplifiers in transmitting RF signals has many applications, including but not limited to radiotelephone communications systems. In a cellular radiotelephone communications system, there are multiple fixed site transceivers located throughout a geographic area for providing radio communications in its surrounding coverage area, referred to as a cell. Each fixed site transceiver is an interface between the line telephone system and portable and mobile radiotelephones located in its cell. The fixed site transceivers and radiotelephones communicate by sending and receiving RF signals to each other.
In frequency division multiple access (FDMA) radiotelephone systems, each requesting radiotelephone is allocated a radio channel (an RF transmit frequency and an RF receive frequency) for the duration of the ensuing communications with the fixed site transceiver. During a phone call, the radiotelephone transmitter would turn-on and remain on the RF transmit frequency for the entire duration of the phone call. Thus, the radiotelephone transmitter and subsequently the RF power amplifier of the radiotelephone only need be turned on once and turned off once for each phone call. Likewise, the transmitter of the fixed site transceiver also remains on for the duration of a phone call. Since the transmitters in FDMA radiotelephone systems are only turned on at the beginning of a call and turned off at the end of a call, the speed at which the power amplifier turns on or off may be relatively slow.
In time division multiple access (TDMA) radiotelephone systems such as the Global System for Mobile Communications (GSM) radiotelephone system currently in use in Europe, 8 radiotelephones can share a single TDMA channel since there are 8 available time slots that are repeated in successive frames. Each radiotelephone is allocated one time slot of a TDMA channel. During the assigned time slot, the radiotelephone ramps up its power amplifier to the proper frequency and output power level, transmits the desired information, and then ramps down its power amplifier so as not to disturb or interfere with the other users sharing the same TDMA channel. Due to the short length of each time slot (e.g. 577 microseconds), it is necessary to accurately control and shape the ramp up and ramp down of the power amplifier output over a wide temperature range. In GSM radiotelephone systems, a time mask and a spectral frequency mask for the ramp up and ramp down of the power amplifier output has been stringently specified by GSM Recommendations 05.05 (Subsections 4.2.2 and 4.5.2, and Annex 2). If the GSM recommendations are not strictly followed, telephones calls on adjacent TDMA channels may be subjected to interference in both time and frequency.
A power amplifier system capable of meeting the GSM recommendations is disclosed in U.S. Pat. No. 5,150,075. However, this power amplifier system relies heavily on a stable RF signal detecting circuitry that is not susceptible to variations in temperature and the power supply voltage. Accordingly, there is a need for an improved RF signal detecting circuitry that is highly stable over a wide range of variations in both temperature and power supply voltage.